A main factor of deterioration of transmission characteristics of surface waves along a transmission path is currently multi-path interference. An OFDM (orthogonal Frequency Division Multiplexing) transmission system resistant to this-multi-path interference is becoming a focus of attention in recent years. This OFDM transmission system is a system of multiplexing a multitude (several tens to several hundreds) of mutually orthogonal digital modulated waves in a certain signal section.
A conventional OFDM communication apparatus performs time-frequency conversion on a reception signal through an FFT circuit, carries out a complex multiplication of pilot symbols contained in the reception signal with known signals, and thereby obtains a channel estimation value. Then, the conventional OFDM communication apparatus carries out a complex multiplication of the channel estimation value with information OFDM symbols, and thereby compensates the information OFDM symbols for propagation distortion. Then, the OFDM symbols compensated for propagation distortion are subjected to error correction by an error correction circuit to obtain an information bit string, which reception data.
When long information is transmitted by the conventional OFDM communication apparatus above, as shown in FIG. 1, channel estimation pilot symbols (hatching area) are inserted at predetermined intervals in information OFDM symbols to follow variations in momentarily changing propagation path characteristics. That is, as shown in FIG. 2, the conventional OFDM communication apparatus compensates information OFDM symbols 1 to n using a channel estimation value obtained with pilot symbol A and compensates information OFDM symbols n+1 to 2n using a channel estimation value obtained with pilot symbol B.
However, in the case where such long information is transmitted, it is necessary to frequently insert known signals such as pilot symbols in order to follow time-variations of the propagation path characteristic. Thus, the conventional OFDM communication apparatus has a problem that the transmission efficiency decreases when long information is transmitted.
To solve such a problem, the present inventor previously proposed an OFDM communication apparatus and channel estimation method that would adaptively perform channel estimations using a decision value of the reception signal as a known signal. This makes it possible to maintain a low error rate by adaptively following time-variations of the transmission path characteristics without reducing the transmission efficiency even when long information is sent and there are great time-variations in the propagation path characteristic.
However, with the OFDM communication apparatus and channel estimation method above previously proposed by the present inventor may have problems as shown below when a residual phase error exists. The “residual phase error” refers to a phase error due to a frequency offset that has escaped being compensated by carrier frequency offset compensation and phase noise of a frequency synthesizer.
That is, the OFDM communication apparatus and channel estimation previously proposed by the present inventor adaptively updates a channel estimation value using a signal obtained by re-coding the reception signal subjected to error correction or a signal obtained by applying a hard decision to the reception signal compensated for propagation distortion and at the same time compensates for a residual phase error. However, since the amount of time variation of the residual phase error is greater than the amount of time variation of the phase error due to a variation of the propagation path characteristic, it is necessary to compensate for the residual phase error only using a newly estimated channel estimation value in order to adaptively update the channel estimation value and at the same time estimate/compensate the residual phase error.
However, when the residual phase error is compensated only using the newly estimated channel estimation value, the error of the channel estimation value will increase in the case where there is an error in the error-corrected information bit or hard-decided information symbol. Furthermore, compensating for the residual phase error by only using the newly estimated channel estimation value will make it impossible to ignore an estimation error due to disturbance such as additive noise. Therefore, preventing deterioration of the reception characteristic will require the channel estimation value to be updated using past information.
However, in the case where compensation of propagation distortion using the past channel estimation value is followed by estimation/compensation of the residual phase error using a pilot carrier, it may be impossible to follow phase variations due to a residual phase error with rapid time variations, advancing a phase rotation too far to estimate the residual phase error.
Furthermore, when there is a large error of compensation for propagation distortion by a pilot carrier, the phase variation of the pilot carrier may be added to the residual phase error to be estimated. If the residual phase error is estimated/compensated under this condition, the amount of phase variation of the pilot carrier varies from one subcarrier to another, generating an error in the estimation value of the residual phase error and causing deterioration of the reception characteristic.